Garmin 396 Reception Problems Solved!

Well, as many of you can remember, I have posted my saga about
intermittent reception problems with my Garmin GPSMAP 396 from day one.

The Narco Nav/Coms or the splitters that are attached to them appear to
be the culprit (both the MK-12D AND Nav 122). I could always select the
harmonic freqs that would send the Garmin out to lunch, but it was not a
constant problem. Sometimes a freq that would tank the Garmin on one day
would be fine the next. This had me doubting the radio stack.

I found that minute changes in the antenna position on the glareshield
would alter the problem dramatically. Sometimes, moving it 1/4 inch
would do it. 8 sats up around 80% to nothing. Zip. Just by moving the
antenna a little bit. Even if the thing was only receiving it would
tank. Local area ILS freqs would bomb the unit (109.5). Turns out that
the default nav freq when you power up the Garmin will lose sat lock. On
the ground, it is fine. Take off and climb and the antenna will move a
wee bit due to the full throttle and high deck angle. Lost sat
reception, but not every time. If the antenna moves just the right way,
it will receive although at a MUCH lower signal strength and only 3 or 4
sats. Move it just a touch and it will degrade enough to lose lock,
maybe down to 0 strength on all sats.

I first thought that the problem of lost sat lock was due to LOW signal
strength and switched to a higher gain antenna. Switching to the higher
gain antenna actually made the problems worse. With it, many more radio
freqs tank the Garmin. Both on COM transmit and NAV receive.

It appears that mounting the antenna up high on the windshield may be
the permanent cure. I cannot justify the ridiculous cost of an external
antenna. $300- $400 for a twenty buck antenna is nuts.

I'll report back if this does the trick. Anyone know of a nifty little
"shelf" I can mount in the Cherokee up high? Maybe something sliding
into the windshield trim plastic?
Mike

I'd be happy to sell you a Garmin GA56 remote antenna. It is in excellent
condition. They were used with the straight Garmin GNS-430. When I upgraded
to the 430W, it came with a different antenna.

$68.....free shipping in the US.

Karl
"curator"

"Mike Spera" wrote in message
...
Well, as many of you can remember, I have posted my saga about
intermittent reception problems with my Garmin GPSMAP 396 from day one.

The Narco Nav/Coms or the splitters that are attached to them appear to be
the culprit (both the MK-12D AND Nav 122). I could always select the
harmonic freqs that would send the Garmin out to lunch, but it was not a
constant problem. Sometimes a freq that would tank the Garmin on one day
would be fine the next. This had me doubting the radio stack.

I found that minute changes in the antenna position on the glareshield
would alter the problem dramatically. Sometimes, moving it 1/4 inch would
do it. 8 sats up around 80% to nothing. Zip. Just by moving the antenna a
little bit. Even if the thing was only receiving it would tank. Local area
ILS freqs would bomb the unit (109.5). Turns out that the default nav freq
when you power up the Garmin will lose sat lock. On the ground, it is
fine. Take off and climb and the antenna will move a wee bit due to the
full throttle and high deck angle. Lost sat reception, but not every time.
If the antenna moves just the right way, it will receive although at a
MUCH lower signal strength and only 3 or 4 sats. Move it just a touch and
it will degrade enough to lose lock, maybe down to 0 strength on all sats.

I first thought that the problem of lost sat lock was due to LOW signal
strength and switched to a higher gain antenna. Switching to the higher
gain antenna actually made the problems worse. With it, many more radio
freqs tank the Garmin. Both on COM transmit and NAV receive.

It appears that mounting the antenna up high on the windshield may be the
permanent cure. I cannot justify the ridiculous cost of an external
antenna. $300- $400 for a twenty buck antenna is nuts.

I'll report back if this does the trick. Anyone know of a nifty little
"shelf" I can mount in the Cherokee up high? Maybe something sliding into
the windshield trim plastic?
Mike

I had that exact same problem with my new 496. On my BFR we did a
couple of ILS approaches as the last thing we did. Since I never use my
nav radios otherwise the frequency was left on the last localizer we
used. Then I noticed the GPS wouldn't work. It would work until I
started the plane then absolutely nothing. As soon as you shut the
plane down the GPS fires up with 12 satellites. This went on for one
week. I took the 496 to the avionics shop. The owner of the shop came
out to the plane with me with another 496. In 5 minutes we figured out
the problem. As soon as we shut off the Mk12D with the localizer
frequency the GPS's worked. Turn it back on and both GPS's failed.
Switch nav freq's and it works again. So we know it's in the one radio
on the nav side and only on certain freq's. I rerouted the external
antenna to the pilots side of the glare shield and this solves the
problem too. The panel mount GPS sits right on top of this radio and is
unaffected. The LOC freq that gave me trouble was 111.5. The other LOC
on 110.3 doesn't cause any problems.

Mike Spera wrote:
Well, as many of you can remember, I have posted my saga about
intermittent reception problems with my Garmin GPSMAP 396 from day one.

The Narco Nav/Coms or the splitters that are attached to them appear to
be the culprit (both the MK-12D AND Nav 122). I could always select the
harmonic freqs that would send the Garmin out to lunch, but it was not a
constant problem. Sometimes a freq that would tank the Garmin on one day
would be fine the next. This had me doubting the radio stack.

On Sep 19, 9:27 am, Newps wrote:
... The LOC freq that gave me trouble was 111.5.

This is a common problem with the 12D. see this previous post:
http://tinyurl.com/342cbu

On Sep 19, 5:33 pm, mikem wrote:
On Sep 19, 9:27 am, Newps wrote:

... The LOC freq that gave me trouble was 111.5.

This is a common problem with the 12D. see this previous post:http://tinyurl.com/342cbu

Follow up: In the old thread, Fong posted that the LO injection on the
12D is low-side, and the first IF is 17MHz. That puts the LO at
115.5-17=98.5. The sixteenth harmonic of 98.5 is 1576MHz, which
apparently blocks the GPS.

On Tue, 18 Sep 2007 18:06:25 -0500, Mike Spera
wrote:

Well, as many of you can remember, I have posted my saga about
intermittent reception problems with my Garmin GPSMAP 396 from day one.

The same reason my 196 loses reception, from Garmin's support database

"
Question: Why is my GPS losing satellites in the middle of my flight?
Answer:

Portable GPS units have the ability to experience loss of satellite
reception as a result of RF interference caused by a variety of
sources. These sources can be as simple as a portable MP3 Player used
for in flight entertainment to a ground based air traffic control
radar antenna on an airport. When these devices are powered on and are
in close proximity to the portable GPS or antenna, the GPS may lose
satellite acquisition as a result. Another common source of RF
interference is aircraft communication and navigation radios. When
certain radios are tuned to a specific frequency there is the
potential for enough RF interference to be released that the Portable
GPS unit will experience loss of satellite reception as though the
signals were being jammed. An article in the Aviation Consumer, dated
February 15th, 1994 has outlined a list of aviation communication
radios and frequencies that may cause a portable GPS unit to lose
satellite reception in the aircraft. This information is listed below.

Radio Frequencies That May Jam GPS Receivers.

Transmit 131.285 and 121.186 Receive

Narco MK 12D/E Com 810/811, Nav 824/825 Com 131.220 and 119.285 Nav
115.464 and 109.672

King KX 155/165 Com 131.820 and 119.885 Nav 116.128 and 109.564

King KX 170/175 Com 122.285 and 130.186 Nav 113.651

Collins Microline Com 132.720 and 120.785 Microline Nav 116.028 and
109.464

Notes: KX 155/165 transmitting on 118.15 ws shown to jam an external
mounted antenna. Narco MK 16 tuned to any 115 or 109 Nav channel was
shown to jam a hand held GPS. Narco MK 12D/E and Nav 824/825, if not
wired with memory keep alive, will default to 115.5 MHz in the active
channel and will jam any GPS receiver.
Last modified on: 08/30/2007

"

On Tue, 18 Sep 2007 18:06:25 -0500, Mike Spera
wrote:

Well, as many of you can remember, I have posted my saga about
intermittent reception problems with my Garmin GPSMAP 396 from day one.

The Narco Nav/Coms or the splitters that are attached to them appear to
be the culprit (both the MK-12D AND Nav 122). I could always select the
harmonic freqs that would send the Garmin out to lunch, but it was not a
constant problem. Sometimes a freq that would tank the Garmin on one day
would be fine the next. This had me doubting the radio stack.

I found that minute changes in the antenna position on the glareshield
would alter the problem dramatically. Sometimes, moving it 1/4 inch
would do it. 8 sats up around 80% to nothing. Zip. Just by moving the
antenna a little bit. Even if the thing was only receiving it would
tank. Local area ILS freqs would bomb the unit (109.5). Turns out that
the default nav freq when you power up the Garmin will lose sat lock. On
the ground, it is fine. Take off and climb and the antenna will move a
wee bit due to the full throttle and high deck angle. Lost sat
reception, but not every time. If the antenna moves just the right way,
it will receive although at a MUCH lower signal strength and only 3 or 4
sats. Move it just a touch and it will degrade enough to lose lock,
maybe down to 0 strength on all sats.

I first thought that the problem of lost sat lock was due to LOW signal
strength and switched to a higher gain antenna. Switching to the higher
gain antenna actually made the problems worse. With it, many more radio
freqs tank the Garmin. Both on COM transmit and NAV receive.

It appears that mounting the antenna up high on the windshield may be
the permanent cure. I cannot justify the ridiculous cost of an external
antenna. $300- $400 for a twenty buck antenna is nuts.

I'll report back if this does the trick. Anyone know of a nifty little
"shelf" I can mount in the Cherokee up high? Maybe something sliding
into the windshield trim plastic?
Mike

We had a similar problem with a Skyforce II GPS which failed when some
NAV frequencies were selected. Checking the satellite signal strength,
on the GPS, confirmed it dropped to zero when the NAV receiver was
tuned to some frequencies. The GPS receiver was being blocked.

Whilst we do have Narco MK12D+ that was not the problem. The DME
(Narco IDME 891) is slaved to the NAV so the fault would clear when
the DME was turned off.

The fault was cleared by fitting an external GPS antenna but before
blaming the DME it should be noted that the Skyforce II GPS was the
problem. It is an old design and the receiver is not as good as the
replacement Skyforce IIIc which works very well.

I'll report back if this does the trick. Anyone know of a nifty little
"shelf" I can mount in the Cherokee up high? Maybe something sliding
into the windshield trim plastic?
Mike

I used velcro to secure the antenna to the upper left corner of the
plastic sun visor, then ran the wire down inside the window trim.

Paul kgyy wrote:
I'll report back if this does the trick. Anyone know of a nifty little
"shelf" I can mount in the Cherokee up high? Maybe something sliding
into the windshield trim plastic?
Mike


I used velcro to secure the antenna to the upper left corner of the
plastic sun visor, then ran the wire down inside the window trim.

O.K. Great idea. 99% of the time, the visor is stowed in the "up"
position. I can mount it on top of the visor in a way that swinging the
visor down just inverts the antenna. Will it work inverted, or sideways?
Thanks,
Mike

Mike Spera wrote:

Paul kgyy wrote:

I'll report back if this does the trick. Anyone know of a nifty little
"shelf" I can mount in the Cherokee up high? Maybe something sliding
into the windshield trim plastic?
Mike



I used velcro to secure the antenna to the upper left corner of the
plastic sun visor, then ran the wire down inside the window trim.

O.K. Great idea. 99% of the time, the visor is stowed in the "up"
position. I can mount it on top of the visor in a way that swinging the
visor down just inverts the antenna. Will it work inverted, or sideways?
Thanks,
Mike
Probably not. The antenna is a patch antenna backed up by a metal
ground plane. If you turn it upside down it will not be facing the
satellites, so if it works at all your signals will be very weak.
Sideways it might be OK if the satellite is visible from that side of
the airplane.